A wavelength multiplexing apparatus includes: a dividing unit to divide a multiplexed optical signal into a plurality of groups each including a plurality of channels; a designating unit to designate, for each group, an arbitrary channel as a designated channel and to designate an adjacent channel in a predetermined frequency direction; an acquisition unit to acquire a first optical level at a central wavelength in the designated channel and a second optical level at a wavelength at which the designated channel and the adjacent channel interfere with each other; a comparison unit to compare a determined ratio of the second optical level to the first optical level with a reference ratio of a third optical level to the first optical level; and an adjustment unit to adjust a wavelength spacing between the designated channel and the adjacent channel in the group, based on a result of the comparison.